As known, light emitters such as for example LEDs are semiconductor light sources with a narrow band emission that can be manufactured to emit in the ultraviolet (UV), visible or infrared regions of the spectrum.
LEDs are used as indicator lamps in many devices, and are increasingly used for lighting. Among LEDs, white LEDs represent a promising, high-efficiency technology for illumination.
To generate white light for general illumination applications, the narrow spectral band of a LED's emission must be converted into white light, or two or more discrete emissions must be mixed.
The conversion of the light emitted by LEDs into white light is performed by phosphor-based LEDs, which are obtained by coating a LED, generally a blue or UV LED, with a phosphor material of different colors selected depending on the color of the original LED, to convert the monochromatic light from the blue or UV LED to an apparent white light, which however does not exhibit a broad band in the visible region.
Mixing different discrete emissions is performed by multi-colored white LEDs, which are obtained by combining individual LEDs emitting different primary colors, generally red, green, and blue.
LEDs are also known which generate white light using a combination of phosphor conversion and color-mixing.
However, on the one side, it is difficult to maintain consistent-quality white light due to natural variations in LED wavelength or in the phosphor material used in phosphor based LEDs.
In addition, prior art white LEDs developed so far exhibit a sharp blue emission leading generally to cool lighting, which in the art is referred to as “blue-light hazard” since it is uncomfortable and hazardous to eyes. Blue light, in fact, due to glare and dazzle effects, may cause eyestrain, retinal injury and fatigue and may disrupt sleep patterns.
Furthermore, the typically available highly emissive phosphors include expensive and environmentally toxic metals, such as silver, cadmium, germanium or rare-earth elements.
In this regard, there have been attempts to provide more stable and efficient and less toxic photoluminescent materials, as described in Hayakawa et al., “White light emission from radical carbonylterminations in aluminosilicate (Al2O3—SiO2) porous glasses with high luminescence quantum efficiencies”, Applied Physics Letters, Vol. 82, No. 18, May 5, 2003. However, the sol-gel derived glasses of aluminosilicate compositions described therein are excited at an excitation length which is too far from the visible region.
On the other side, the mechanism involved in the electro-optical devices used to control the blending and diffusion of the different colors in multi-colored white LEDs renders this type of white LEDS of limited use.